Japanese Patent Application Laid-Open (kokai) No. 2013-89428 discloses a method for inspecting the dielectric strength of an insulator of a spark plug. In this inspection method, in an assembly obtained by assembling a metal shell, a metal terminal, a center electrode, and an insulator, a voltage is applied between the metal terminal and the metal shell. When a defect such as cracking is present in the insulator, a current flows via the defect, and thus a voltage that can be applied is decreased. Therefore, when the applied voltage exceeds a predetermined value, the dielectric strength of the insulator is determined as good, and when the applied voltage is equal to or less than the predetermined value, the dielectric strength of the insulator is determined as not good. In this inspection method, application of the voltage is performed in a state where a rear end portion of the insulator is covered with a cylindrical insulating member having an electrical insulation property. Accordingly, a phenomenon in which a current flows between the metal terminal and the metal shell through the vicinity of the rear end of the insulator (also referred to as flash over phenomenon) can be inhibited, and thus a situation where dielectric strength cannot be inspected can be avoided.
In this respect, a flash over phenomenon can be inhibited more as the insulating member is more strongly brought into close contact with the insulator. However, when the insulating member is strongly brought into close contact with the insulator, it becomes difficult to detach the insulating member from the insulator after inspection, so that there is a possibility that smooth progress of the inspection is hindered.
The present specification discloses a technology that allows inspection of an insulator of a spark plug to smoothly progress while inhibiting a flash over phenomenon during the inspection.